In order for coating procedures of substrates by means of evaporative processes, or by means of vapor deposition methods, respectively, to be regulated and controlled it is necessary for the amount or the volumetric flow of the evaporating material that settles on the substrate to be measured. An evaporative process and consequently the entire coating process can be sufficiently monitored or controlled, respectively, by means of measurements of this type.
For some materials and elements, for example for selenium, it is known in principle for a proportion of the evaporated material to be extracted from a vapor chamber, typically from a vacuum chamber, and to be supplied to a vibration plate, typically to an oscillating crystal. The amount of the material that is deposited on the vibration plate leads to a modification of the resonance frequency of the vibration plate, said modification being electronically detectable. To this extent, the shift in the resonance frequency is a measure for the mass and the thickness of the layer accumulating on the vibration plate. To this extent, a volumetric flow of the evaporated material that has to be measured can be measured during the ongoing coating procedure by way of vibration plates of this type.
The vibration plate is to be disposed so as to be spaced apart from the vacuum chamber. Said vibration plate is typically to be coupled to the vacuum chamber in a gas-conducting or vapor-conducting manner by way of an extraction section. When extracting a vapor jet from the vacuum chamber it has to be guaranteed that an amount of the material vapor that is sufficient for a stable measuring signal is extracted. However, it can arise that a condensation temperature of the extracted vapor jet is undershot across the extraction section. Across the longitudinal extent of the extraction section this can lead to part of the material vapor that is propagated by way the extraction section condensing. Of course, such undesirable condensation leads to a distortion of the measuring signal.
It is to be furthermore noted that known vibration plates, typically in the form of quartz or other piezoelectric crystals, when subjected to continuous deposition of material, gradually lose their ability to vibrate in a required resonance range. To this extent, the service life of vibration plates of this type is heavily dependent on the amount of material that accumulates on the vibration plate.
Measuring installations which have a plurality of vibration plates which are impingeable in a sequential manner with the material vapor to be measured in order for the service life of known measuring assemblies to be prolonged are known. A plurality of such vibration plates herein is typically disposed on a rotatably mounted support of a turret measuring head. Individual vibration plates that are disposed on the support can be sequentially held in the extracted vapor jet or be positioned at that end of the extraction section that faces away from the vacuum chamber, respectively, by rotating the support.
Known turret measuring heads are often sealed only in an inadequate manner such that vibration plates that have not yet been used, prior to being employed are subjected to unintentional coating. On account thereof, not only are the measuring results distorted but the reproducibility of measurements, in particular when vibration plates are changed, can be compromised on account thereof.
The present invention is therefore based on the object of providing an improved measuring assembly for measuring the layer thickness of a layer that is capable of being applied to a substrate by means of a vapor deposition method. To this extent, the measuring assembly is intended to provide an inflow of vaporous or gaseous material, respectively, to the vibration plate that is as minor as possible yet stable and steady. The measuring assembly is intended to supply particularly precise measuring results and at the same time enable a comparatively long lifespan of the vibration plates that are to be used for measuring the layer thickness.
It is therefore a further objective to avoid or to suppress as far as possible any unintentional coating of vibration plates that are not yet located in the vapor jet, so as to enable more exact and more reproducible measurements in particular when vibration plates are changed, in order to ultimately simplify regulating and controlling of evaporative processes.